This invention relates to a novel composition to be used in preparing microporous polyvinylidene fluoride polymer membranes useful for liquid separations and other purposes. This invention further relates to a process utilizing the composition to prepare a microporous polyvinylidene fluoride membrane. This invention still further relates to a microporous polyvinylidene fluoride polymer membrane useful for liquid separations, for liquid-gas interchanges, and other uses.
Polyvinylidene fluoride type polymer membranes have long been used in membrane separation processes such as microfiltration, ultrafiltration, dialysis, membrane distillation, membrane stripping and the like. Membranes made from polyvinylidene fluoride polymers have good solvent resistance, are easily processed, and are low in cost. Typically, polyvinylidene fluoride polymer membranes are prepared by extruding, molding, or casting the membranes from blends containing a polymer, a solvent, and an optional non-solvent. Solvents are compounds in which the polymer substantially dissolves at the membrane fabrication temperature. Non-solvents are compounds in which the polymer is substantially insoluble at the membrane fabrication temperature. Solvents which have been used to prepare polyvinylidene fluoride polymer membranes include dimethylformamide, dimethyl acetamide, N-methyl-2-pyrrolidinone, and dimethyl sulfoxide. Non-solvents which have been used to prepare polyvinylidene fluoride polymer membranes include ethylene glycol, propylene glycol, methanol, propanol, water, and maleic acid. Generally, residual amounts of such solvents and non-solvents cannot be left in the membranes because they cause unacceptable contamination of the fluids being treated. Avoiding such contamination is particularly important in the treatment of biological fluids, such as blood or water. The solvents and non-solvents are therefore typically completely removed during membrane fabrication by extensive leaching. Once the solvents and non-solvents are removed from the membranes, they present problems of disposal or extensive repurification before reuse.